The purpose of the proposed investigation is to provide strenth-duration curves for ventricular defibrillation using bipolar catheter electrodes located in the right heart chambers. Strenth-duration curves will be generated for square (rectangular) waveforms, and for trapezoidal waveforms having various tilts. The dependence of the efficacy of a trapezoidal defibrillation pulse upon the tilt in the waveform will be quantitated. It is known that a rectangular pulse (zero tilt) will defribrillate ventricles with a lower current than is required for a simple capacitor discharge (100 percent tilt) of the same duration. It is not known how much tilt can be allowed before a significant increase in current is required to restore the efficacy of the waveform. Defribrillators that produce very little tilt must contain a large number of capacitors; defibrillators that have a high degree of tilt must produce large currents (because of the nature of strength-duration curve of cardiac muscle), and hence, must charge the capacitors to very high voltages. At the present time, there exists no data to indicate the best choice between these two extremes, although such defibrillators are now commercially available. The effect of subject size and heart size upon the current and energy necessary to defibrillate a trapezoidal pulse will also be determined. The goal of the proposed research is to provide a data base that will allow optimization of design and construction of implantable (automatic) defibrillators.